Stratigraphy and palaeoclimatic significance of Late Quaternary loessâ•fipalaeosol sequences of

42 Pages·2017·2.3 MB·English

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USGS Staff -- Published Research US Geological Survey
2003
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llooeessss––ppaallaaeeoossooll sseeqquueenncceess ooff tthhee LLaasstt IInntteerrggllaacciiaall––GGllaacciiaall ccyyccllee
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Daniel R. Muhs
US Geological Survey, [email protected]
Thomas A. Ager
US Geological Survey, [email protected]
E. Arthur Bettis III
University of Iowa, [email protected]
John McGeehin
US Geological Survey
Josh M. Been
US Geological Survey
See next page for additional authors
Follow this and additional works at: https://digitalcommons.unl.edu/usgsstaffpub
Part of the Earth Sciences Commons
Muhs, Daniel R.; Ager, Thomas A.; Bettis, E. Arthur III; McGeehin, John; Been, Josh M.; Begét, James E.;
Pavich, Milan J.; Stafford, Thomas W. Jr.; and Stevens, De Anne S. P., "Stratigraphy and palaeoclimatic
significance of Late Quaternary loess–palaeosol sequences of the Last Interglacial–Glacial cycle in
central Alaska" (2003). USGS Staff -- Published Research. 181.
https://digitalcommons.unl.edu/usgsstaffpub/181
This Article is brought to you for free and open access by the US Geological Survey at DigitalCommons@University of
Nebraska - Lincoln. It has been accepted for inclusion in USGS Staff -- Published Research by an authorized
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AAuutthhoorrss
Daniel R. Muhs, Thomas A. Ager, E. Arthur Bettis III, John McGeehin, Josh M. Been, James E. Begét, Milan
J. Pavich, Thomas W. Stafford Jr., and De Anne S. P. Stevens
This article is available at DigitalCommons@University of Nebraska - Lincoln: https://digitalcommons.unl.edu/
usgsstaffpub/181
ARTICLE IN PRESS
QuaternaryScienceReviews22(2003)1947–1986
Stratigraphy and palaeoclimatic significance of Late Quaternary
loess–palaeosol sequences of the Last Interglacial–Glacial cycle in
central Alaska
Daniel R. Muhsa,*, Thomas A. Agera, E. Arthur Bettis IIIb, John McGeehinc,
Josh M. Beena, James E. Bege!td, Milan J. Pavichc, Thomas W. Stafford Jr.e,
De Anne S.P. Stevensf
aUSGeologicalSurvey,MS980,Box25046,FederalCenter,Denver,CO80225,USA
bDepartmentofGeoscience,UniversityofIowa,IowaCity,IA52242,USA
cUSGeologicalSurvey,MS955,NationalCenter,Reston,VA20192,USA
dDepartmentofGeologyandGeophysics,UniversityofAlaska,Fairbanks,AK99775-5780,USA
eStaffordResearchLaboratories,Boulder,CO80301,USA
fAlaskaDivisionofGeologicalandGeophysicalSurveys,Fairbanks,AK99775,USA
Received21July2001;accepted6August2002
Abstract
LoessisoneofthemostwidespreadsubaerialdepositsinAlaskaandadjacentYukonTerritoryandmayhaveahistorythatgoes
back3Ma.Basedonmineralogyandmajorandtraceelementchemistry,centralAlaskanloesshasacompositionthatisdistinctive
fromotherloessbodiesoftheworld,althoughitisquartz-dominated.CentralAlaskanloesswasprobablyderivedfromavarietyof
rocktypes,includinggranites,metabasaltsandschists.Detailedstratigraphicdataandpedologiccriteriaindicatethat,contraryto
earlystudies,manypalaeosolsarepresentincentralAlaskanloesssections.Theburiedsoilsindicatethatloesssedimentationwas
episodic,oratleastratesofdepositiondecreasedtothepointwherepedogenesiscouldkeepaheadofaeolianinput.AsinChina,
loess deposition and pedogenesis are likely competing processes and neitherstops completelyduring either phase of the loess/soil
formation cycle. Loess deposition in central Alaska took place before, and probably during the last interglacial period, during
stadials of the mid-Wisconsin period, during the last glacial period and during the Holocene. An unexpected result of our
geochronological studies is that only moderate loess deposition took place during the last glacial period. Our studies lead us to
concludethatvegetationplaysakeyroleinloessaccumulationinAlaska.Factorsfavouringloessproductionareenhancedduring
glacial periods but factors that favour loess accumulation are diminished during glacial periods. The most important of these is
vegetation;borealforestservesasaneffectiveloesstrap,butsparselydistributedherbtundradoesnot.Thus,thickaccumulationsof
loess should not be expected where tundra vegetation was dominant and this is borne out by modern studies near the treeline in
central Alaska. Much of the stratigraphic diversity of North American loess, including that found in the Central Lowlands, the
Great Plains, and Alaska is explained by a new model that emphasizes the relative importance of loess production factors versus
loess accumulation factors.
Published byElsevier Science Ltd.
1. Introduction Hovan et al., 1989), but it is also important in setting
boundary conditions in atmospheric general circulation
Loess is receiving much attention today in the models, or AGCMs (Kutzbach et al., 1993). Loess
palaeoclimate community. Not only is loess thought to records provide one of the few ways to test reconstruc-
be a terrestrial equivalent to the foraminiferal oxygen tion of paleowinds that are simulated by AGCMs
isotoperecordindeep-seasediments(Kuklaetal.,1988; (Muhs and Bettis, 2000). In addition, however, dust is
now also being considered as having a role as a climate
forcing mechanism itself (Tegen et al., 1996; Mahowald
*Corresponding author. Tel.:[phone]; fax: [phone]-
etal.,1999;KohfeldandHarrison,2000,2001;Harrison
5349.
E-mailaddress:[email protected](D.R.Muhs). et al., 2001). Thus, there is considerable interest in loess
0277-3791/03/$-seefrontmatter PublishedbyElsevierScienceLtd.
doi:10.1016/S0277-3791(03)00167-7
ARTICLE IN PRESS
1948 D.R.Muhsetal./QuaternaryScienceReviews22(2003)1947–1986
notonlyasaclimaterecordarchive,butforitseffectson Loess is widely distributed in Alaska, and is areally
the overall planetary radiation balance. the most extensive surficial deposit in the state (Fig. 1).
In Europe and in parts of the North American Based on studies by Westgate et al. (1990), the Alaskan
midcontinent (see Bettis et al., 2003), the stratigraphic loess record may extend back to B3.0Ma. Alaskan
record is straightforward because, in general, loess loess has been studied for a century, beginning with an
deposits indicate glacials or stadials while palaeosols investigationbySpurr(1898)intheYukonRivervalley.
indicate interglacials or interstadials (Ruhe, 1983). In Numerousgeomorphicprocesseshavebeencitedforthe
China, interpreting loess records is more complex, originofuplandsiltinAlaska,includingfluvial,marine,
because loess is deposited during both glacials and lacustrine, estuarine deposition, and accumulation as
interglacials (Kukla et al., 1988; Hovan et al., 1989; residuumfromunderlyingbedrock.However,itwasthe
Verosubetal.,1993;Dingetal.,1994;Xiaoetal.,1995). pioneering work of Pe!we! (1955) that brought forth a
Therefore, thick unaltered loess deposits represent high variety of data that now clearly points to an aeolian
rates of loess deposition and palaeosols represent low origin as loess. Since that time, the stratigraphy and
rates. In Alaska, although loess is thought to be largely palaeoenvironmental significance of loess in Alaska has
glaciogenic, the stratigraphic record may be somewhat beenstudiedinanumberofregions,butparticularlythe
analogous to that of China, because loess deposition in central interior portion (Figs. 2, 3) of the state (Pe!we!,
Holocene time is well documented (Pe!we!, 1951, 1975a, 1955, 1975a, b; McCulloch et al., 1965; Hamilton et al.,
b; Bege!t, 1990; Reger et al., 1996; Manley et al., 2001). 1983, 1988; Bege!t, 1988, 1990, 1991, 1996, 2001; Bege!t
176° 168° 160° 152° 144°
C
C TI Barrow O
R C
A E
Chukchi Sea A
N
66° Prudhoe Bay
SGW
SPeenwianrsdula BROOKS SL
RANGE
EP
KY
EL
Nome A
D
A
62° SMI River HHCR FCXH AN
Fairbanks C
n
YuKkuoskokwim River ASKA RANGSEC TaDnaJna River D
L
A
MV
Anchorage
58° AHKLUN
Bering Sea MOUNTAINS Kenai
KP
Gulf of Alaska
0 100 200
KILOMETRES
Mass accumulation
Loess
rate (MAR) locality
Fig.1. MapshowingthedistributionofloessinAlaska(adaptedfromHopkins,1963,fortheSewardPeninsulaandPe!we!,1975afortherestofthe
state) and localities where mass accumulation rates were calculated. Abbreviations: SGW, Sagwon; SL, Slope Mountain; EL, Egg Lake; EP,
Epiguruk;KY,Koyukuk;SMI,SaintMichaelIsland;HH,HalfwayHouse;CR,ChatinikaRiver;FX,Fox;CH,ChenaHotSpringsRoad;SC,Shaw
CreekFlats;DJ,DeltaJunction;MV,MatanuskaValley;KP,KvichakPeninsula.
ARTICLE IN PRESS
D.R.Muhsetal./QuaternaryScienceReviews22(2003)1947–1986 1949
148° 146° 144° 142°
BROOKS RANGE
68° 68°
Chandalar River
River Porcupine
YUKON Riv er Fort Yukon
Yu k o n FLATS 66°
66° Circle
EXPLANATION
Yu
k
o
n Loess
River
Moraine
YUKON-TANANA
Summer
UPLAND wind
Fairbanks
direction
Eielson
Nenana ABiar sFeorce Net annual
wind
64° direction
64° BDieglta J uDneclttaion
Geochemical
sample locality
ALASKA RANGE Summit Tanana River 0 50
Lake
148° 146° 144° 142° KILOMETRES
Fig.2. MapshowingthedistributionofloessincentralAlaska,morainesthoughttodatefromthelastglacialperiod,geochemicalsamplelocalities
intheYukonFlatsarea,andnetresultantannualandsummerwinddirectionsforwindvelocitiesX5m/s.LoessdistributionfromPe!we!etal.(1966),
Williams(1962)andminornewmappingbytheauthors;morainepositionsfromKarlstrometal.(1964);resultantwinddirectionscalculatedbythe
authors.
and Hawkins, 1989; Bege!t et al., 1990; Westgate et al., interpretations about the nature of the loess strati-
1990;EdwardsandMcDowell,1991;Bergeretal.,1994; graphic record in the region. Although loess in Alaska
Berger et al., 1996; Pe!we! et al., 1997; Sher et al., 1997; hasbeenstudiedfordecades,thereislittleornomention
Preece et al., 1999; Vlag et al., 1999; McDowell and of palaeosols in these deposits (e.g., Pe!we!, 1955, 1975a,
Edwards, 2001; Muhs et al., 2001a; Lagroix and b;Pe!we! etal.,1997).Morerecentstudies,however,such
Banerjee, 2002). as those of Hamilton et al. (1988), Bege!t and Hawkins
In this paper, we test several hypotheses about the (1989), Bege!t et al. (1990), Bege!t (1990), Hamilton and
palaeoclimatic significance of the loess record in central Brigham-Grette (1991), Vlag et al. (1999), McDowell
Alaska. The first of these involves two conflicting and Edwards (2001), Muhs et al. (2001a) and Lagroix
ARTICLE IN PRESS
1950 D.R.Muhsetal./QuaternaryScienceReviews22(2003)1947–1986
148°00' 147°30' 1996), and near the Ahklun Mountains in southwestern
65°00' Alaska (Manley et al., 2001). Dust storms are common
pQ pQ
at present in the Delta Junction and Matanuska Valley
pQ areas of Alaska (Pe!we!, 1951; Trainer, 1961). In the
pQ
stratigraphicrecord,itisnotknownwhethermoreloess
pQ
Qal deposition occurredduring thelast glacial period or the
Holocene, but the question is an important one for
pQ climate modeling (Mahowald et al., 1999; Kohfeld and
pQ
pQ Harrison, 2000, 2001; Harrison et al., 2001).
Finally, linked climate-biome-biogeochemical models
pQ BIRCH arenowcapableofsimulatingprobabledustsources,on
HILL
aglobalbasis,duringthelastglacial period (Mahowald
GOLD
HILL Qal et al.,1999; Kohfeld and Harrison, 2000). Simulation of
probabledustsourcesisbasedonidentificationofthose
Chena River
regions whose earth system properties (strong winds, a
FAIRBANKS
decreased intensity of the hydrologic cycle, and a
Qal
pQ
Qal diminished vegetation cover) favour dust entrainment.
In the models produced by Mahowald et al. (1999) and
Tanana
reviewed by Kohfeld and Harrison (2000, 2001) and
River
Qal 0 5 Harrisonetal.(2001),itisinferredthatmanyhigh-latitude
KILOMETRES Qal regions in the Northern Hemisphere, including Alaska,
64°45'
were more important dust sources during the last glacial
EXPLANATION
Qal Alluvium period than during the Holocene. In addition to new
data from the Fairbanks area presented here, we review
Upland loess
published stratigraphic and geochronologic data that
Vreawlloeyrk leode slose asnsd allow us to calculate mass accumulation rates (MARs)
pQ Pbered-rQocukaternary for the last glacial period and the Holocene for various
parts of Alaska.
Geochemical
sample locality
Fig.3. Mapshowingthedistributionofuplandandvalleyloessinthe
Fairbanks area (equivalent to the Fairbanks D-2 quadrangle) and 2. Methods
geochemicalsamplelocalities.GeologyredrawnfromPe!we! (1958).
2.1. Analytical techniques
and Banerjee (2002) report the existence of numerous Detailed stratigraphic and pedologic studies of four
palaeosols, indicating major periods of little or no loess thick loess sections were conducted in the Fairbanks
deposition and relative landscape stability. Similar area. Studies of modern Alaskan soils by Muhs et al.
findings have also been reported for adjacent parts of (2000a) show that certain properties are likely to be the
Yukon Territory in Canada (Fraser and Burn, 1997). best indicators of pedologic development with which
We test these conflicting ideas by using established one can identify palaeosols in loess sections. These
pedologic criteria for the existence of palaeosols in four include field properties such as colour and sequence of
Alaskan loess sections that we have studied in detail in genetic horizons, organic matter concentrations, and
the Fairbanks area. concentrationsoftotalP.Organicmatterconcentrations
New stratigraphic and geochronologic studies of the for most sections were determined by a modified
loess record are also used to test the concept that most Walkley–Black method (for concentrations less than
loess deposition in Alaska represents glacial periods. B7%) and by loss-on-ignition at 550(cid:1)C for B12h (for
ThismodelisaclassiconeforAlaska(Pe!we! etal.,1965, concentrations greater than B7%) (see Allison, 1965).
1975a)aswellasforthemidcontinentofNorthAmerica For one section (Chena Hot Springs Road), all organic
(Ruhe, 1983) and Europe (Smalley and Leach, 1978). matter concentrations were estimated by loss-on-igni-
However, because glaciers still exist in Alaska, glacio- tion(950(cid:1)CforB1h).Muhsetal.(2000a)showedthat,
genic silt production and subsequent aeolian entrain- in non-calcareous loess and palaeosols, there is a very
ment occur today. It is not surprising, therefore, that high positive correlation between estimates of organic
there is abundantevidenceof significant Holocene loess mattermadebytheWalkley–Blackandloss-on-ignition
depositionininteriorAlaskanearDeltaJunction(Pe!we!, methods, even for samples with less than 7% organic
1975a), around Fairbanks (Hamilton et al., 1983, 1988; matter. Major element concentrations were determined
Bege!t, 1990), on the Kenai Peninsula (Reger et al., by wavelength-dispersive X-ray fluorescence, where
ARTICLE IN PRESS
D.R.Muhsetal./QuaternaryScienceReviews22(2003)1947–1986 1951
precision is typically 70.01%. Trace element geochem- isons were made to published data on glass chemistry
istry,usingimmobileelementssuchasTiandZr,isoften fortephrasininteriorAlaskareportedbyWestgateetal.
useful for assessing the uniformity of loess source (1983,1985),Bege!tetal.(1991),Preeceetal.(1992),and
sediments (see Muhs et al., 1990 for an example of this Berger et al. (1996).
approach for far-traveled dust). For most samples, Extraction of 10Be from bulk soil and loess samples
concentrationsofTiandZrweredeterminedbyenergy- follows the method outlined by Curry and Pavich
dispersive X-ray fluorescence; for some samples, Ti was (1996). A calibrated, dissolved spike of 9Be was added
determined by wavelength-dispersive X-ray fluores- to each ground sample, mixed with NaCO and heated
3
cence. In a comparison of loess of the Fairbanks area to 1200(cid:1)C in quartz crucibles. The resulting glass beads
with loess of the Yukon Flats area, both selected major were processed chemically to extract a purified BeO
elements(K,Ca,Ti)andtraceelements(Sr,Ba,Zr)were powder. Ratios of 10Be/9Be in the BeO powders were
determined by energy-dispersive X-ray fluorescence. measuredbyacceleratormassspectrometryatLawrence
Particle size analysis of loess and palaeosols was Livermore National Laboratory. Age estimates were
conducted by a combination of sieve and pipette made from 10Be concentrations, horizon thicknesses,
methods. Samples were pre-treated with H O to bulk densities (volume measured by the tube method;
2 2
destroy organic matter, with HCl to remove carbonates weights determined on 110(cid:1)C-oven-dry samples), and
(although none are typically present) and with Na- estimates of global production rates (Tables 3 and 4).
hexametaphosphate to enhance dispersion. Sands (par-
ticles with diameters >53mm) were separated from silts 2.2. Organic matter and phosphorus distribution in
andclaysbywetsieving;abundancesofcoarsesilts(53– modern soils and palaeosols
20mm), fine silts (20–2mm) and clays (o 2mm) were
determined by settling and pipette analysis. Laboratory The question of whether or not palaeosols exist in
precision for pipette analysis was routinely monitored Alaskan loess sections can be answered by use of
with each suite of sample runs by analysis of an Iowa morphological and chemical properties that are char-
loess standard maintained by the Department of acteristic of modern soils. Three time-tested pedologic
Agronomy, Iowa State University. methods,supportedbystudiesofmodernAlaskanloess-
Chronology is based on geochemical identification derived soils (Muhs et al., 2000a), are employed in this
and correlation of dated tephras, accelerator mass study. The most useful morphological properties are
spectrometric (AMS) radiocarbon dating of wood, colour, texture, structure, and evidence of subsurface
charcoal, and humic acids from soil organic matter illuviation of mobile soil constituents such as clay or
found in loess and buried soil horizons, and 10Be age iron. However, many soil morphological properties can
estimates based on the profile ‘‘inventory’’ method be removed by erosion or obscured after burial by
(Curry and Pavich, 1996). AMS radiocarbon ages diagenetic overprints and cryoturbation. Furthermore,
(Table 1) were determined on wood, charcoal and weak soil development may be difficult to discern in
humicacidextractionsfromorganicmatterinpalaeosol loess sequences. Therefore, we also used distribution of
A horizons. For humic acids, detailed extraction steps organic matter and phosphorus as tests of our field
are in Abbott and Stafford (1996) and McGeehin et al. identifications of possible palaeosols based on soil
(2001), but include the following: (1) removal of morphology. Distribution of organic matter and phos-
material coarser than 63mm by sieving, (2) removal of phoruswill not necessarily allow identification ofpartly
carbonates by a 6N HCl leach, followed by decantation erodedsoils,butcanhelpconfirmthepresenceofpoorly
and centrifuge washing, (3) a 1–5% KOH extraction at developedsoilsthathaveweakmorphologicexpression.
roomtemperature,followedbycentrifugationtoremove
clays (B30min at high speed), and (4) filtration of the 2.3. Geochronology of central Alaska tephras
supernatant solution through a 0.45 micron teflon
millipore filter, followed by acidification. The samples Several tephras that have been studied in the Fair-
were then dried and graphitized and concentrations of banks area are important in loess stratigraphy, and
14C were measured by accelerator mass spectrometry at some have been either directly or indirectly dated by
Lawrence Livermore National Laboratory. Radiocar- numerical methods. The Old Crow tephra has been
bon ages were converted to calendar years using the identified extensively over interior Alaska and Yukon
program of Stuiver et al. (1998) for samples less than Territory (Westgate et al., 1983, 1985; Bege!t, 1991;
B20,000 14C yr BP and using the closest calendar-year Preeceetal.,1992,1999).Westgate(1988,1989)usedthe
ages available from Voelker et al. (2000) for samples isothermal-plateau fission-track method for analysis of
older than B20,000 14C yr BP (Table 1). the Old Crow tephra from the Holitna Lowland in
Tephras were identified by major-element geochem- southwestern Alaska and the Halfway House section
istry, using a microprobe, determined on glass fractions west of Fairbanks. He reported ages of about
isolated from bulk tephra samples (Table 2). Compar- 150,000720,000yr for two analyses from the Holitna
ARTICLE IN PRESS
1952 D.R.Muhsetal./QuaternaryScienceReviews22(2003)1947–1986
Table1
Acceleratormassspectrometric(AMS)radiocarbonagesofhumicacidextractions,charcoalandmacrofossilsfromcentralAlaskanloess
Locality Field# Depthin Lab# LLNL# Material FractionModern Radiocarbon Calendar
section(m) age(yr) age(yr)
DeltaJunction AK-314-a 0.5–0.6 NSRL-2807 CAMS-24347 Wood 0.983070.0052 140750 294(142)0
AK-314-b 0.9–1.0 NSRL-2808 CAMS-24348 Wood 0.958470.0049 340750 508(356)294
AK-314-c 1.1–1.2 NSRL-2809 CAMS-24349 Wood 0.925670.0043 620740 662(596)539
AK-314-d 2.4–2.5 NSRL-2810 CAMS-24350 Wood 0.788970.0056 1900760 1988(1848)1707
AK-314-e 2.9 NSRL-2811 CAMS-24351 Wood 0.729270.0042 2540750 2753(2728)2363
AK-314-f 3 NSRL-2812 CAMS-24352 Humicacids 0.718070.0061 2660770 2919(2761)2622
ShawCreek AK-320A 0.33–0.40 NSRL-2813 CAMS-24353 Humicacids 0.979470.0057 170750 305(175)0
Flats AK-320B 0.90–0.93 NSRL-2814 CAMS-24354 Humicacids 0.306770.0027 9490780 11156(10714)10507
HalfwayHouse AK-202 0.15 NSRL-1904 CAMS-16090 Wood 0.979570.0066 170760 310(175)0
Nenana- AK-204 0.26 NSRL-1905 CAMS-16085 Humicacids 0.970870.0072 240760 461(294)1
Fairbanksarea AK-206 0.36 NSRL-1906 CAMS-16082 Humicacids 0.981070.0075 150770 310(144)0
AK-208 0.53 NSRL-1907 CAMS-16088 Wood 0.984870.0074 120770 302(97)0
AK-226 3.98 NSRL-1908 CAMS-16089 Humicacids 0.022270.0013 30,5807480 B33,960
AK-228 4.33 NSRL-1909 CAMS-16094 Humicacids 0.017470.0013 32,5307620 B35,780
AK-230 4.6 NSRL-2864 CAMS-24377 Humicacids 0.026870.0010 29,0807300 B33,520
AK-240 6.26 NSRL-1910 CAMS-16095 Humicacids 0.005370.0013 42,16072030 B43,280
GoldHill, AK-12 1.8 NSRL-1898 CAMS-16093 Humicacids 0.021370.0013 30,9107500 B34,240
Fairbanksarea AK-15 2.21 NSRL-1899 CAMS-16092 Humicacids 0.015970.0013 33,2807680 B36,360
AK-18 2.46 NSRL-1900 CAMS-16091 Humicacids 0.028670.0013 28,5607380 B32,300
AK-24 3.36 NSRL-1901 CAMS-16084 Humicacids 0.007870.0014 39,02071420 B41,740
AK-51 7.6 NSRL-1903 CAMS-16083 Humicacids 0.004270.0013 43,89072510 B43,880
BirchHill, AK-714 3.34 NSRL-2815 CAMS-24355 Humicacids 0.032370.0009 27,5807220 B30,180
Fairbanksarea AK-717 3.65 NSRL-2816 CAMS-24356 Humicacids 0.013770.0008 34,4807500 B40,520
AK-719 3.82 NSRL-2817 CAMS-24357 Humicacids 0.021970.0009 30,6807320 B34,000
AK-719 3.82 NSRL-2818 CAMS-24358 Charcoal 0.005670.0009 41,60071200 B43,000
AK-730 7.76 NSRL-2819 CAMS-24359 Humicacids 0.008370.0008 38,5307810 B41,540
ChenaHot AK-900A 0.44 WW-3178 CAMS-72995 Charcoal 0.358070.0012 8250730 9400(9167)9090
SpringsRoad, AK-900B 0.98 WW-3179 CAMS-72996 Charcoal 0.319670.0014 9160740 10473(10352)10220
southsidecut,
Fairbanksarea
ChenaHot Eastofmaintrench:
SpringsRoad, AK-909C 0.4 WW-3181 CAMS-72998 Charcoal 0.353670.0015 8350740 9486(9402)9153
northsidecut, AK-909yyy 1.1 WW-3250 CAMS-74439 Humicacids 0.296970.0011 9750740 11226(11176)11136
Fairbanksarea AK-909xxx 1.3 WW-3249 CAMS-74438 Humicacids 0.281770.0015 10,180750 12327(11813)11444
AK-909B 1.7 WW-3248 CAMS-74437 Humicacids 0.018170.0005 32,2107240 B36,220
Maintrenchsection:
AK-910A 1.26 WW-3278 CAMS-75000 Humicacids 0.023970.0005 30,0107170 B33,780
AK-912A 1.42 WW-3279 CAMS-75001 Humicacids 0.024070.0005 29,9507160 B33,660
AK-909zzz 2.8 WW-3251 CAMS-74440 Humicacids 0.010570.0004 36,5907340 B40,840
AK-937 6.35 WW-3280 CAMS-75002 Humicacids 0.009970.0005 37,0507390 B41,000
AK-944 7.1 WW-3281 CAMS-75003 Humicacids 0.007470.0004 39,3807490 B41,740
AK-944A 7.1 WW-3182 CAMS-72999 Charcoal o0.000970.0003 >55,900 >55,900
Lowland samples, and ages of 120,000720,000yr and partial bleach method, Berger et al. (1994) reported
160,000730,000yr for two analyses from the Halfway bracketingTLagesofX140,000730,000yr(loessbelow
Housesamples.Theseagesareinbroadagreementwith the Old Crow tephra) and 110,000732,000yr (loess
recentTLagedeterminationsofbracketingloessaround above the Old Crow tephra) at the Halfway House
the Old Crow tephra reported by Berger et al. (1994, locality. At Birch Hill, where the Old Crow tephra is
1996), although it should be noted that TL ages older also found, Berger et al. (1996) reported TL ages of
than B100,000yr are open to question. Using the 144,000722,000yr (loess below the tephra) and
ARTICLE IN PRESS
D.R.Muhsetal./QuaternaryScienceReviews22(2003)1947–1986 1953
Table2
GeochemistryoftephrasfromtheHalfwayHouse,GoldHillandChenaHotSpringsRoadloesssections(allinw%)
Locality n NaO MgO AlO SiO Cl KO CaO TiO Fe O Total Tephra
2 2 3 2 2 2 2 3
(UAF#) correlation
HalfwayHouse
AK-232A
(Act2236) 11 4.25 0.5 14.38 74.42 0.14 2.03 1.61 0.34 2.32 100 Halfway
sd 0.13 0.04 0.23 0.32 0.01 0.07 0.09 0.09 0.13 House
AK-254
(HHOC-2002) 15 3.75 0.31 13.27 75.37 0.18 3.61 1.28 0.35 1.88 100 OldCrow
sd 0.15 0.03 0.25 0.3 0.04 0.13 0.16 0.11 0.27
AK-254 14 3.71 0.28 13.07 75.19 0.29 3.67 1.44 0.29 2.06 100 OldCrow
(HHOC-1990) 0.16 0.09 0.09 0.37 0.04 0.11 0.1 0.03 0.1
sd
GoldHill
AK-48
(Act2004) 4 4.53 0.55 14.24 71.74 0.1 3.55 1.8 0.69 2.79 100 Dome
sd 0.24 0.04 1.12 0.34 0.06 0.25 0.49 0.42 0.47
AK-68A
(Act2241) 29 4.71 1.38 15.71 67.86 0.05 2.18 3.61 0.71 3.79 100 Unnamed
sd 0.42 0.14 0.48 0.65 0.02 0.18 0.29 0.14 0.29 andesitic
tephra
ChenaHotSpringsRd
AK-944/945
(Act2529) 17 4.58 0.51 14.62 71.22 0.11 3.74 1.8 0.54 2.88 100 Dome
sd 0.11 0.05 0.17 0.35 0.03 0.1 0.09 0.14 0.17
128,000722,000yr (loess above the tephra). At Eva as occurring in a trench, distinct from the one in the
Creek, another locality near Fairbanks, Berger and presentstudy,atGoldHill(Preeceetal.,1999).NewTL
Pe!we!(2001)reportedaTLageof136,000720,000yrfor ageestimatesatthislatterlocalitysuggestthatitisolder
loess above the Old Crow tephra but below the Eva than 76,000710,000yr (Berger and Pe!we!, 2001). At
Forest Bed of last-interglacial age. Based on our own Halfway House, it occurs above the Old Crow tephra,
studiesatEvaCreek,wedonotagreethattheOldCrow but its stratigraphic relation to the Dome tephra is
underlies the Eva Forest Bed; our interpretation of the unknown.
stratigraphyisthattheOldCrowtephraisfoundwithin
the forest bed (Muhs et al., 2001a). Nevertheless, we
agreefromthemultipleanalysesbydifferenttechniques, 3. Mineralogy and geochemistry of Alaskan loess
along with their associated errors (all given above at
only 1-sigma uncertainties), that the Old Crow tephra The mineralogy and geochemistry of loess deposits
hasalikelyageof160,000–120,000yr.However,wealso provide important information about loess particle
believe that the simple averaging practice to yield a origins, source sediments, geomorphic processes and
reported age of B140,000yr is misleading. palaeoclimate. Loess is often perceived to be a rather
The Dome tephra is undated by any direct means. It uniform deposit of silt-sized particles dominated by
occurs stratigraphically above the Old Crow tephra at quartz (Pye,1987).However, recent studieshave shown
theEvaCreeklocality(seePe!we!etal.,1997;Muhsetal., that the geochemistry of loess from different regions is
2001a), and is stratigraphically below palaeosols with variable, reflecting diverse origins (Gallet et al., 1998).
ages of B39,000yr (likely a minimum age) at Gold Hill Evenwithinagivenregion,suchastheNorthAmerican
and>55,900yrattheChenaHotSpringsRoadsections midcontinent, loess compositions can be quite different
(see below). Thus, the Dome tephra is younger than (Muhs and Bettis, 2000).
160,000–120,000yr but older than B56,000yr. The There are very few data on the mineralogical and
Halfway House tephra is the least studied of all the chemical composition of Alaskan loess deposits. Thus,
namedtephrasfoundintheFairbanksarea.Itoccursat part of our effort has been to characterize the
Halfway House (see discussion below), and is reported geochemistry of central Alaskan loess using both major
1
Table3 95
Horizonthicknesses,10Bemeasurements,bulkdensity(BD)measurementsandageestimatesforloessandpalaeosolsattheHalfwayHousesection 4
Sample Horizon Depth(cm) Horizon Measured10Be 10Be,corrected Bulkdensity SourceofBD 10Be,corr./vol. 10Be,corrected 10Beindepth Ageinterval Cumulative
thickness (108atoms/g) (108atoms/g) (g/cm3) measurement (108atoms/cm3) (108atoms/cm2 interval/ (yr) age(yr)
(cm) depthinterval) productionrate
AK-208 5Ob 53 20 2.08 1.928 0.36 Measured 0.69408 13.8816 10.678 1067.816 1067.816
AK-209 5Eb 67.5 9 2.76 2.608 1.08 Measured 2.81664 25.34976 19.5 1949.982 3017.798
AK-210 5Bwb 80 16 2.29 2.138 1.43 Measured 3.05734 48.91744 37.629 3762.881 6780.679
AK-211 5C 98 20 0.66 0.508 1.39 Measured 0.70612 14.1224 10.863 1086.339 7867.018
AK-212 5C 118 20 0.33 0.178 1.48 Measured 0.26344 5.2688 4.053 405.292 8272.31
AK-213 5C 138 20 0.27 0.118 1.4 Measured 0.1652 3.304 2.542 254.154 8526.464
AK-214 5C 158 20 0.28 0.128 1.4 Estimated 0.1792 3.584 2.757 275.692 8802.156
AK-215 5C 178 20 0.56 0.408 1.4 Estimated 0.5712 11.424 8.788 878.769 9680.926
D
AK-216 5C 198 20 0.53 0.378 1.4 Estimated 0.5292 10.584 8.142 814.154 10495.08 .R
AK-217 5C 218 20 0.57 0.418 1.4 Estimated 0.5852 11.704 9.003 900.308 11395.388 .
M
AK-218 5C 238 20 0.48 0.328 1.4 Estimated 0.4592 9.184 7.065 706.462 12101.85
u
AK-219 5C 258 20 0.51 0.358 1.4 Estimated 0.5012 10.024 7.711 771.077 12872.927 h
s
AK-220 5C 278 20 0.5 0.348 1.4 Estimated 0.4872 9.744 7.495 749.539 13622.466 e
t
AK-221 5C 298 20 0.55 0.398 1.4 Estimated 0.5572 11.144 8.572 857.231 14479.697 a
AK-222 5C 318 20 0.47 0.318 1.4 Estimated 0.4452 8.904 6.849 684.923 15164.62 l./ A
AK-223 5C 338 20 0.38 0.228 1.4 Estimated 0.3192 6.384 4.911 491.077 15655.697 Qu R
AK-224 5C 358 20 0.35 0.198 1.4 Estimated 0.2772 5.544 4.265 426.462 16082.159 ate T
AK-225 5C 379.5 23 0.38 0.228 1.21 Measured 0.27588 6.34524 4.881 488.096 16570.254 rn IC
AK-226 6Ab 397.5 12 1.53 1.378 1.17 Measured 1.61226 19.34712 14.882 1488.24 18058.495 ary L
AK-227 6C 413 20 0.87 0.718 1.24 Measured 0.89032 17.8064 13.697 1369.723 19428.218 S E
AAKK--222289 77ABwb1b 444373 280 22..4133 21..297788 11..324 EMsetiamsuarteedd 32..035723562 2447..44270216 1386..758157 13867581..467943 2243905789..398162 cienc IN
AK-230 7Bw2b 460.5 19 2.53 2.378 1.34 Measured 3.18652 60.54388 46.572 4657.223 29615.609 e
R P
AK-231 7Bw3b 473 6 0.36 0.208 1.34 Estimated 0.27872 1.67232 1.286 128.64 29744.249 e
v R
AK-232 7C 484 16 0.5 0.348 1.2 Measured 0.4176 6.6816 5.14 513.969 30258.218 ie
w E
AK-233 7C2 505 26 0.55 0.398 1.24 Measured 0.49352 12.83152 9.87 987.04 31245.259 s S
AK-234 7C3 540 44 0.93 0.778 1.24 Estimated 0.96472 42.44768 32.652 3265.207 34510.466 22 S
AK-235 7C4 573 22 0.7 0.548 1.24 Estimated 0.67952 14.94944 11.5 1149.957 35660.423 (2
AK-236 7C5 591.5 15 0.55 0.398 1.24 Estimated 0.49352 7.4028 5.694 569.446 36229.869 00
3
AK-237 7C6 603.5 9 0.89 0.738 1.24 Estimated 0.91512 8.23608 6.335 633.545 36863.414 )
AK-238 7C7 613 10 0.9 0.748 1.24 Estimated 0.92752 9.2752 7.135 713.477 37576.891 19
AK-239 7C8 621.5 7 1.24 1.088 1.24 Estimated 1.34912 9.44384 7.264 726.449 38303.341 47
–
AK-240 8Ab 626.5 3 3 2.848 1.11 Measured 3.16128 9.48384 7.295 729.526 39032.867 1
9
AK-241 8Bwlb 629.5 3 1.79 1.638 1.46 Estimated 2.39148 7.17444 5.519 551.88 39584.747 8
6
AK-242 8Bw2b 633 4 2.69 2.538 1.46 Estimated 3.70548 14.82192 11.401 1140.148 40724.895
AK-243 8Bw3b 636.5 3 1.28 1.128 1.46 Measured 1.64688 4.94064 3.8 380.049 41104.945
AK-244 8C 648 20 1.08 0.928 1.4 Measured 1.2992 25.984 19.988 1998.77 43103.714
AK-245 8C 668 20 1.02 0.868 1.33 Measured 1.15444 23.0888 17.761 1776.062 44879.777
AK-246 8C 688 20 1.07 0.918 1.35 Measured 1.2393 24.786 19.066 1906.616 46786.392
AK-247 8C 708 20 0.83 0.678 1.35 Estimated 0.9153 18.306 14.082 1408.154 48194.547
AK-248 8C 728 20 0.65 0.498 1.35 Estimated 0.6723 13.446 10.343 1034.308 49228.855
AK-249 8C 748 20 0.71 0.558 1.35 Estimated 0.7533 15.066 11.589 1158.923 50387.778
AK-250 8C 768 20 0.93 0.778 1.35 Estimated 1.0503 21.006 16.158 1615.847 52003.625
AK-251 8C 788 20 1.05 0.898 1.35 Estimated 1.2123 24.246 18.651 1865.077 53868.702
AK-252 8C 808 20 0.95 0.798 1.35 Estimated 1.0773 21.546 16.574 1657.385 55526.087
AK-253 8C 828 20 0.72 0.568 1.35 Estimated 0.7668 15.336 11.797 1179.693 56705.78
AK-255 8C 871 20 0.33 0.178 1.35 Estimated 0.2403 4.806 3.697 369.692 57075.473
AK-256 8C 891 20 0.41 0.258 1.35 Estimated 0.3483 6.966 5.358 535.846 57611.319